This invention relates to reworkable electronic semiconductor components, including multi-chip modules (xe2x80x9cMCMsxe2x80x9d), that incorporate electrical heaters integrally within the component structure to produce the heat necessary to soften or weaken the bond of the component to the printed wiring board to which the component is attach, allowing removal of the component from a printed wiring board for rework. More particularly, the invention relates to a new heater structure for the electronic semiconductor component that is fault tolerant to current-interrupting breaks as may be formed or produced in any of the heaters. The invention is applicable to substrate-to-printed wiring board attachments that employ adhesive bonds, such as found in the thermoset adhesive lead type components, or that employ reflow solder bonds, such as found in ball grid array lead-less type components.
The present invention improves upon the invention of Berkely et al presented in U.S. Pat. No. 6,031,729, granted Feb. 29, 2000 entitled xe2x80x9cIntegral Heater for Reworking MCMS and Other Semiconductor Componentsxe2x80x9d (hereafter the xe2x80x9cBerkely et al ""729 patentxe2x80x9d) assigned to TRW Inc., the assignee of the present invention. In a broader aspect, the invention improves upon electrical heater systems as may be applied in other ways than presented in the foregoing patent by incorporating circuits that provide fault tolerance to current-interrupting breaks in the electric heaters of an electric heater system for an electronic component that avoids disruption of heating.
A principal application of the present invention is with reworkable Multi-Chip Modules, such as described in the cited Berkley et al ""729 patent. Multi-Chip Modules (xe2x80x9cMCMsxe2x80x9d) perform a variety of electronic functions, and are finding increasing use in sophisticated electronic applications, particularly airborne and space-borne application. By definition, an MCM contains two or more semiconductor die or chips, as variously termed, and ancillary electrical components, assembled in a single enclosed package, that together comprise an electronic circuit function. The semiconductor chips contain the micro-miniature integrated circuits, such as processors, amplifiers, memory, and the like.
In one type of MCM structure, the semiconductor chips and components are supported on a common base, consisting of an integral multi-layer printed wiring structure, referred to as the substrate. Often that substrate is formed of ceramic, an electrical insulator that is rigid, allows for plated-on conductors of the finest widths and spacing with the greatest accuracy and is able to maintain a hermetic seal. Metallic conductors printed on various layers of the substrate, and metallic vias through the layers, serve to electrically connect the semiconductor chips to each other and to the external interfaces of the MCMs.
The foregoing elements are contained together in a single enclosed four-sided package, often hermetically sealed, that serves as a protective housing for the semiconductor chips and associated components. The ceramic substrate, being hermetic, serves as the bottom wall to the module. A metal wall, or seal ring, is brazed to the substrate around the perimeter, encompassing the components and a lid welded to the top surface of this seal ring hermetically seals the components inside. A number of electrical contacts or leads extend out the four sides of the MCM to provide external electrical input-output connections to the MCM.
In practice MCMs are generally installed upon a printed wiring board, much larger in area than an MCM, that contains the electrical interconnections between the MCMs and other components thereon. The larger wiring board is typically constructed of a material such as glass-epoxy or glass-polyimide, a less expensive and lower quality material than the ceramic of the substrate. For airborne and space applications, MCMs are typically bonded to the printed wiring boards. Bonding enhances thermal conductivity to the MCM, and isolates mechanical loads from the input-output connections of the MCM, which promotes longer product life. A variety of adhesives, such as thermosetting epoxies or thermoplastics, and solder are available to provide the bonding.
To bond the MCM in place, as example, a layer of thermally sensitive adhesive is applied to either the underside surface of the MCM, or directly to the surface of the printed wiring board at the location to which that component is to be placed. With the MCMs and all other components for that circuit board properly positioned, the board is then placed in an oven and the temperature raised to cure or reflow the adhesive, attaching the MCMs and other components in place. When removed from the oven and cooled down to room temperature the MCMs are firmly attached to the printed wiring board.
Solder is another known thermally sensitive adhesive material used to fasten parts together. A second known technique for fastening the MCM to the circuit board is the solder ball grid array. Instead of incorporating electrical leads extending from the side of the MCM package and using a separate adhesive for fastening the MCM to the circuit board, as in the foregoing structure, the electrical leads are instead formed by electrical vias extending through the multiple layers of substrate to the underside surface of the MCM package. At the underside the terminal end of those vias typically appear by design arranged in regular rows and columns. Minute solder balls or solder columns, different geometry""s for the dab of solder collectively referred to herein as solder balls, are formed at the terminal ends of those vias on the underside of the substrate.
In assembly, the MCM package is placed upon the printed wiring board, the latter of which contains solder pads that mate with the solder balls on the MCM package and the temperature is raised above the solder eutectic at which the solder reflows. When cooled, the solder solidifies and provides a firm mechanical connection that fastens the MCM package to the printed wiring board as well as completing the electrical connections to printed circuitry on that wiring board. The foregoing connection apparatus and technique is well known.
If failed components were detected during subsequent electrical testing of the assembled board, the failed components needed to be removed from the printed wiring board for repair or replacement. The problem in reworking MCM""s, whether fastened to the circuit board by regular adhesives or with a solder ball grid array, is recognized as endemic to other large size electronic semiconductor components as well, even those that contain only a single physically large semiconductor chip. As those skilled in the art recognize, the more modern semiconductor chips are growing in physical size as more and more circuit functions are expected to be packed within a single die even in commercial devices, such as cellular telephones. As a consequence large numbers of very fine closely spaced wires are required to interface to the semiconductor die. Because the wires must all extend into the die they are necessarily physically small in width and must be packed closely together, typically one mil in diameter separated by a two mil space. However, conventional printed circuit board technology typically provides semiconductor die interface connections with no less than a four mil separation.
To resolve the apparent physical incompatibility in spacing requirements, the approach taken has been to mount the semiconductor chip onto an intermediate xe2x80x9cinterposerxe2x80x9d substrate, which is often formed of ceramic material. The printed wiring formed on the substrate fans out from the microscopic spacing at the location of the semiconductor die or chip to the wider spacing and wider wiring required by the conventional printed circuit board.
That electronic semiconductor assembly is then mounted onto the printed circuit board. The electrical leads from the assembly substrate are soldered to the mating solder pads on the printed circuit board, or, should the substrate instead employ a solder ball grid array, the solder balls are soldered to the mating solder pads formed on the printed circuit board. As in the case of the earlier described MCMs, in the foregoing arrangement, viewed in a generic sense, one multi-layer printed circuit board is mounted atop another printed circuit board. The dimension critical wire bonding of the electrical leads to the chips, thus, is accomplished on the ceramic substrate. Interconnect to the printed circuit board is accomplished by soldering the electrical leads from the substrate to mating pads on the conventional circuit board. With such an interposer or intermediate substrate, in retrospect, one recognizes the parallel between the foregoing structure and that of the MCM, earlier described.
Heat was employed to assemble each of the foregoing electronic components; and heat is the means that was typically used to remove an MCM or other thermally bonded unit from the printed wiring board. The difficulty and problems encountered in removing MCM""s from the printed wiring board for rework, particularly in large size MCM""s, those over 1.5 inch in a dimension, using traditional techniques, such as application of a heat gun, is described at some length in the Berkley ""792 patent to which the interested reader may make reference and need not be here repeated.
The Berkely et al ""729 patent describes a new structure by means of which heat may be uniformly applied to the underside of the substrate sufficient to permit detachment of the MCM from the printed wiring board without damage. The reworkable MCM presented in the Berkely et al ""729 patent includes electric heater elements formed in a metallized pattern typically printed on the bottom most internal layer of the multi-layer substrate of the MCM; in effect to form an integral heater assembly. In addition to the multiple layers of the substrate that contains the printed-on metal interconnections for the semiconductors and input-output connections of the MCM, a dedicated bottom layer to the multi-layer substrate contains a number of printed and fired-on resistive conductors, suitably arranged in a pattern, such as a serpentine pattern, each of which serves as a heater. When current is passed through the heater, the resulting I2R losses in the conductor of the heater is evoked as heat. Together, the multiple heaters effectively covers the surface of the bottom layer with heat; and the heat is conducted to the adhesive bond to the printed wiring board. By design, the heat produced is sufficient to weaken the bond between the substrate of the MCM and the printed wiring board, but is insufficient to cause delamination of the multiple layers of the substrate.
The Berkely et al ""729 patent also discloses a preferred embodiment in which the electrical conductors and supporting layer that forms the heater (or heaters) are formed of the same conductor and substrate materials used in the other layers of the multi-layer substrate, such as aluminum oxide and tungsten, respectively, permitting convenient manufacture of the heater as part of a conventional substrate fabrication process.
By incorporating within the structure of the electronic semiconductor components a heater that facilitates removal of that component from its installed adhesive-bonded position on a printed wiring board in the event of a semiconductor component failure, individual electronic semiconductor components may be expeditiously and efficiently removed and replaced. Any necessity for discarding the entire printed wiring board, along with other good electronic components, is avoided, eliminating the expensive procedure of building the entire circuit board assembly anew.
The individual heaters in the MCM described in the Berkely et al ""729 patent are connected electrically in parallel between elongate conductors along a pair of opposed sides of the substrate, as example, along the front and rear sides of the substrate; and each heater is formed of a fine line of metal. In one embodiment, each heater forms a serpentine-like pattern in between the two sides. A large number of such heaters cover the area of the substrate, thirteen in one example of the ""792 patent. As one realizes, if the heater wire of an individual heater in the foregoing structure is broken, that heater cannot conduct electrical current. Since the heat produced by the heater wire is produced by the I2R loss, being unable to conduct current, no heat can be produced; and that produces a heating discontinuity in the substrate that could wholly or partially negate the advantage of the embodiment of the Berkely et al ""729 patent.
A current-interrupting break could be produced during fabrication processing of the substrate layers, as example, should a piece of dust lodge on the substrate during plating. A second possibility for creating a break is due to mishandling during assembly of the MCM. As example, should an assembler inadvertently scratch the substrate on another solid and scrape or cut through a heater line. A third possibility occurs during rework of the MCM, should the technician raise the voltage applied to the heaters to a level that results in too high a current through a heater lead, one or more of the heaters may overheat and, like a fuse, burn out, producing a break in the line. Unless the break is large enough to visually observe, it can only be found by testing. For consistency it would be necessary to electrically test each substrate produced, and that testing procedure takes time and effectively raises the production costs. Irrespective of the underlying reason for a current-interrupting break in a heater, the availability of some means to automatically xe2x80x9cpatch upxe2x80x9d the break or effectively minimize the effect of a break in a heater as would give the MCM a fault tolerant characteristic, and would be of benefit to and improve upon the foregoing combination.
Accordingly, a principal object of the present invention is to provide a reworkable MCM or other electronic semiconductor component that employs an integral heating system for permitting detachment of the component from a printed wiring board with a heater system that is fault tolerant.
And another object of the invention is to eliminate the necessity for testing of the integrity of heaters contained in a reworkable electronic component so as to reduce production cost without detracting from the effectiveness of the heaters, even though one or more of the heaters contains a break.
In accordance with the foregoing objects, the invention incorporates fault tolerance within the integral electric heaters of a reworkable electronic semiconductor component, such as a reworkable multi-chip module, to increase production yield and longevity of the rework feature. Components of the foregoing kind contain a multi-layer substrate to bond to a printed wiring board, and, for rework, the component includes a plurality of electric heaters arranged side by side on a bottom layer of the substrate. When energized with current, the heaters generate sufficient heat to weaken the adhesive or solder bond to the printed wiring board without delaminating the layers of the substrate, allowing the electronic semiconductor component to be pulled away from the printed wiring board for rework. Additional circuitry, specifically a series of buses, is included to automatically route heater current around, that is, bypass any current-interrupting break (or breaks) as may form in any of the electric heaters rendering the heaters tolerant to that kind of fault.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.